1. Field of the Invention
The present invention concerns a method to determine the strength of a magnetic interference field that is caused by an interfering object in an imaging magnetic resonance measurement, and a magnetic resonance system for this purpose.
2. Description of the Prior Art
In imaging magnetic resonance tomography, a contrast agent is frequently used in order to make specific structures in the body of an examination subject visible. It is also possible to introduce tracer particles into the body of the person to be examined, the tracer particles having specific (for example magnetic) properties enabling the particles to be located in the body. Hollow spheres—that can furthermore exhibit a therapeutic effect on the body of the person to be examined—are known for use as tracer particles. For example, islet cells that subsequently produce insulin in the body of the person to be examined can be introduced into the hollow spheres.
The magnetic susceptibility of the tracer particles or the contrast agent often differ significantly from the susceptibility of the material that surrounds the tracer particles in the body of the person to be examined. In an imaging magnetic resonance measurement (data acquisition), the tracer particles thus represent interference points, and thus they can only be imaged with difficulty and lead to image artifacts in the acquired image data. Furthermore, accumulations of the tracer particles in the body of the person to be examined cannot be quantified. The body of the person to be examined may also exhibit additional interference points, for example dental implants or air bubbles that likewise lead to artifacts in the image data. Not only is it difficult to image these interference objects, but the artifacts can overlay surrounding regions, such that no evaluable image data are obtained from these overlaid regions.
There is thus a need not only to obtain an optimally artifact-free imaging of the region to be examined but also to be able to make quantitative statements about the interference object. In particular the determination of the concentration of an accumulation of interference objects and the quantification of the magnetic susceptibility of the interference object or of the material surrounding the interference object and the volume of the interference object are of interest. Such a quantification cannot be achieved with conventional methods.